21．The earthquake and the nuclear proliferation(核扩散) are a ______to Japan as well as the whole world.
A .surprise B.chanllenge C.reaction D.threat
22．We shouldn't complain about being poor—many families are much ________ than we are.
A．worse off B．badly off C．well off D．better off
23．He didn't make ________ clear when and where the meeting would be held.
A．this B．that C．it D．these
